This study is designed to study the neruochemical substrates of the anticonvulsant effect of VNS in patients with partial-onset epilepsy. 1H-MRS will be used to determine local tissue concentrations of NAA, cho,and Cr in the hippocampus and the insular cortex before, and 2 months and 6 months after the start of VNS. Values obtained before and after the start of VNS metabolic/neurochemical systems of the brain.